This invention relates generally to nickel-cadmium storage batteries and more particularly to a process for activating nickel-cadmium cells during their manufacturing process.
Nickel-cadmium (Ni/Cd) batteries are important energy storage devices having many uses including use in communication satellites. In the manufacture of such batteries, it is necessary to activate the nickel-cadmium cell by means of adding a known amount of electrolyte to dry battery cells and repetitive charging and discharging cycles prior to use, a process which has become known as formation cycling. In general, the electrolyte used is an aqueous solution of about 20% to 40% by weight of potassium hydroxide. In addition, in the manufacture of hermetically sealed batteries, it is important that the correct quantity of liquid electrolyte be added to each cell. Overfilling a nickel-cadmium cell, for example, even by a small amount, will cause oxygen pressure build-up due to blockage of pores in the separator by the excess amount of electrolyte. The open pores are needed for oxygen gas passage from the nickel electrode to the cadmium electrode where the oxygen is recombined with charged cadmium. The oxygen gas is evolved at the nickel electrode during the charge half-cycle as a normal parasitic reaction. Such recombination is essential if the build-up of excessive gas pressure within the battery and permanent damage are to be avoided. Underfilling the cell is likewise undesirable since underfilling can result in high internal resistance, reduced cell capacity, and reduced battery service life.
Typical present day nickel-cadmium batteries have separators made from either nylon or polypropylene felt material. In the manufacturing process for such batteries, overfilling of a cell, either by a mistake in estimating the quantity of electrolyte required or by a manufacturing accident, can only be corrected by a laborious process of inverting the battery cells and then charging them in the inverted position whereupon, under the force of gravity, the excess electrolyte flows out through a fluid fill tube of the battery.
Recently, nickel-cadmium batteries of an improved design have been introduced. In these batteries, sometimes referred to as "super" nickel-cadmium batteries, the previously used felt separators have been replaced by a zirconia separator material. However, because of the stronger capillary action exhibited by the zirconia separators, the above-mentioned technique for correcting the overfilling problem has not been found to be effective.
The presently used cell activation technique of adding a known amount of electrolyte to a dry cell has a serious disadvantage in that any impurity--for example, carbonate--introduced into the cell during the dry cell manufacturing steps cannot be removed from the cells as part of the activation process, since the electrolyte cannot be easily removed once it is introduced into the cell.
Another problem arising in the use of present day nickel-cadmium batteries is an observed reduction in cell capacity over a period of storage time. This phenomenon is known as capacity fading.
A principal object of the present invention is to reduce capacity fading of such batteries on storage.
Another object of the present invention is to provide a process which will enable the controlled removal of excess electrolyte from nickel-cadmium batteries and especially from super nickel-cadmium batteries employing separators exhibiting strong capillary action.
Another object of the present invention is to provide a means of flushing the cell with an excess amount of electrolyte and finally leaving the cell with a fresh portion of electrolyte as part of the activation process.
Another object of the present invention is to improve the cell capacity of nickel-cadmium storage batteries.
Other objects and advantages of the present invention will become apparent from the following detailed disclosure and description.